One of the challenges in designing high-speed permanent magnet motors is retention of the magnets on the rotor shaft during high-speed operation. Centrifugal force due to rotation of the rotor causes the magnets to exert an outward (radial) force that must be mechanically restrained by the shaft. Since the centrifugal force is proportional to the radius at the center of mass of the magnet and the square of the angular speed, high speed/high power permanent magnet motor designs are difficult.
Permanent magnet motors and alternators use two approaches to mount magnets to the rotor shaft. The first is called surface mounting wherein adhesives are used to bond the magnets to the surface of the rotor shaft. In many designs, the magnets are additionally retained by strapping high-strength, non-metallic bands around the rotor shaft over the magnets. See, for example, Johnson et al. U.S. Pat. No. 5,801,470 entitled “Rotors with Retaining Cylinders and Reduced Harmonic Field Effect Losses” and Van Dine et al. U.S. Pat. No. 6,603,232 entitled “Permanent Magnet Retaining Arrangement for High Speed Rotors”. The speed and power of permanent magnet motors or alternators with magnets mounted with this first approach are limited due to the limited strength of the adhesives and bindings. The second approach to magnet retention on permanent magnet rotors is embedding in which the magnets are inserted below the surface of the shaft and are held in place by mechanical means. In one such approach, a ring with axial protrusions retains the magnets. See Smith U.S. Pat. No. 6,700,288 entitled “High Speed Rotor”. The ring is attached to the shaft through a dovetail or fir tree fit at each pole on the shaft. With solid rings, the tolerances can stack up between pole attachments causing possible interference with other parts during assembly if the clearances are insufficient. The solid ring is also difficult to machine. A proposed variation of this method involves individual carriers for each magnet instead of a solid ring. See, for example, Weiglhofer et al. U.S. Pat. No. 6,548,932 entitled “Nonmagnetic Magnet Retention Channel Arrangement for High Speed Rotors”.
It is an object according to this invention to provide a magnetic retention system for individual magnets held by permanent magnetic rotors. The invention is especially useful in large permanent magnet motors or alternators and is particularly characterized by eliminating all low permeability materials between the magnets, laminations, and the stator poles.